1. Field of the Invention
This invention relates to devices and methods used to detect the locations at which the intensity of an optical image exceeds a threshold level, and more particularly to phase conjugate resonators employed in optical data processing.
2. Description of the Prior Art
There are many signal processing applications which require the processing of electrical data at a very high rate. The use of optics for these applications is very appealing because of the massive parallelism that can be obtained, i.e., a large amount of information can be processed using a single beam. One application of particular interest is the processing and detection of electronic signals by electro-optic correlation techniques. In these and various other applications it is desirable to employ a binary or threshold type of detection scheme which is sensitive to the optical signal at many different locations in the beam. The object is to detect whether the beam's optical intensity at one or more locations exceeds or falls below a threshold level, rather than to determine the absolute magnitude of the beam intensity at such locations.
For example, one may detect the occurrence of a particular alpha-numeric character by optically correlating a candidate character or a field of characters with every possible character, either in sequence or in parallel. The desired character will produce the highest correlation peak in the output of an optical correlator. In searching over a field of multiple correlations the identification of normalized intensity peaks greater than a specified value thus identifies the character.
The intensity of a correlation peak could be simply detected with an optical detector and fed into an electronic threshold detector whose activated output then identifies a detection event. Unfortunately, neither the expected position of the character nor its associated correlation peak is generally known. Accordingly, a very large number of optical detectors, perhaps numbering in the tens of thousands, might be required to cover the entire field of possible positions. Each of the detectors must have fast temporal responses, especially if the occurrence event is short-lived. Furthermore, all of the detectors must have individual threshold circuits, since in general each detector produces a spurious output. Such outputs occur even with "noise-free" detectors, since large numbers of detectors typically see strong cross-correlation peaks which, though individually falling well below threshold, may in their total output exceed the threshold level (assuming the detector outputs are accumulated by summing over blocks of the outputs). Thus, one conventional approach is to use a large number of individual threshold circuits, one per detector.
At present, the detection of processed images is done with a fast detector array, such as a fast television camera, and complicated electronics are used to average over a set number of frames and compare the intensity to a pre-set threshold value. The equipment required to accomplish this function is complex, expensive and space consuming.